In a transceiver of a communication system, a digital front end (DFE) is known as the circuitry which lies between the baseband processor and the analog baseband/RF circuits. Generally, the DFE comprises components that process a baseband signal for various channels in the digital domain and provide an up-converted digital signal which may undergo additional signal processing. For instance, the up-converted digital signal may utilize techniques such as noise cleaning, crest factor reduction (CFR) and digital pre-distortion of the signal. Finally, the processed signal may be applied to a digital-to-analog converter of the transceiver before entering a power amplifier.
In multi-band communication systems, the up-converted digital signal may comprise carrier-bands separated by a significant frequency bandwidth. Such configuration represents a challenge for signal processing operations such as noise cleaning and crest factor reduction (CFR), since it requires very high sampling rate which is not practical for existing hardware. This is mainly due to the fact that the instantaneous bandwidth of the up-converted digital signal, which is defined as the total bandwidth encompassing all the carriers of the up-converted digital signal, is very wide and that, according to Nyquist sampling theorem, the required sampling rate need to be greater than the instantaneous bandwidth.
A conventional processing method, which focuses on crest factor reduction (CFR) applications, suggests reducing the sampling rate of such multi-band signals by positioning a baseband version of each of the carriers to a frequency separation between adjacent carriers of at least two channel bandwidths, thereafter, combining the position carriers into a composite signal, clipping the composite signal, repositioning each carrier of the clipped signal back into a baseband version centered at zero frequency, and positioning each carrier at its respective center frequency.
Although effective, this method supposes the explicit extraction and repositioning of each carrier of the carrier-bands so as to create the composite signal, thereby requiring the modification of current digital up-conversion units. Furthermore, this method seems to assume that all carrier-bands of the composite signal have the same bandwidth, and therefore does not address cases where carrier-bands of a composite signal have different bandwidths.